1. Field of the Invention
The invention relates generally to circuits for detecting malfunction in amplifiers and more specifically for an improved circuit for detecting amplifier malfunction over a wide range of input signals.
2. Prior Art
In a general sense, detection circuits may be classified into two broad groups. In the first group are circuits which protect a system from transient conditions or disturbances which occur external to the system. Some of the well known transient conditions are; over-voltage, under-voltage, over-current and under-current. The general approach in solving the problems in the first group is to sense the abnormal condition at an external point to the system to be protected and isolating the system once the abnormal condition is sensed. One of the well known procedures is to select an alternate path to ground once the abnormal condition is sensed.
In the second group are circuits which sense malfunction conditions occurring within the system to be protected. In other words, the malfunction is generated within the system rather than a malfunction occurring without the system as in the first group previously mentioned.
The present invention may be useful in systems classified in both the first group and second group, but finds its greatest utility in the systems which are classified within the second group.
In general, the present invention is directed to circuits for sensing the malfunction in an amplifier. More specifically, the invention is directed to circuits for sensing the malfunction in an amplifier which drives the take-up spool and supply spool of a reel-to-reel tape transport. An alternate use is for sensing malfunction occurring in amplifiers which drive the motors in an X-Y coordinate library system.
Reel-to-reel tape transport systems are widely known. In one form a generally cyclindrical mandrel or drum includes a rotating head wheel which carries one or more read/write heads. The magnetic tape engages the mandrel at one point, makes a helical wrap-around at least a portion of the mandrel, and exits the mandrel at a point which is both axially and circumferentially spaced from the entrance point. The angle of helical tape wrap can vary in accordance with design choice, but is usually between 180.degree. and 360.degree.. The head wheel rotates so as to sweep its magnetic heads traversely across the tape. The angle at which the head enters and exits the tape may vary in accordance with design choice, from slightly less than 90.degree. to a small angle, such as 15.degree..
A supply spool and a take-up spool is positioned so as to bring the tape in transducing relationship with the rotating head. Each spool is driven by a motor. The motor is in turn driven by power amplifiers. Due to the accuracy which is required in controlling the tension in the tape, any malfunction in the power amplifiers must be instantaneously detected. Failure to detect malfunction will result in snapping or breaking of the tape.
Various techniques have been used in the prior art for detecting amplifier malfunction or for detecting error within a servo control system. In one scheme a predetermined constant error is generally introduced by conventional means. For example, if a constant load is being driven by the servo system, a torque motor is used to apply a torque opposite to the direction in which the load is moved. The torque is used to displace a potentiometer arm and an electrical error signal is introduced into the system. The error signal is fed back into the system and is monitored. Any deviation from the known error signal is a warning that the system is malfunctioning.
Although the prior art devices embodying the above enumerated scheme operates satisfactorily for the intended purpose, there are several problems which plague these devices. One of the problems is the high cost and bulkiness of these prior art devices. The high cost stems from the fact that in order to introduce the constant error into the system, additional hardware is required; for example, the torque motor and the circuit for converting the torque into an electrical signal. The additional cost of this hardware increases the overall unit cost. Also the additional hardware adds to the overall size and hence, the bulkiness of the unit.
As is well known to those skilled in the art, the present demand is for low cost miniaturized components. With this restraint, the prior art devices are not suitable for several applications.
Another drawback with the prior art servo monitoring scheme is that the load which is driven by the servo system has to be constant. In a servo system where the load is varying, the prior art scheme will not function satisfactorily to detect malfunction in the system. There are several applications wherein the malfunction detector must detect malfunction in a servo system which is driving a variable load. For example, a reel-to-reel servo control system wherein the load (magnetic media) on the take-up spool varies.
Another prior art scheme which has been used to detect amplifier malfunction in a servo controlled system is the so-called random sampling approach. This scheme requires a test run to be undertaken at a predetermined time during normal operation of the system.
Although the random sampling scheme has solved some of the problems which were created by the scheme in which error is introduced into the system, several new problems are created.
In order to perform a test run on the system, the normal operation of the system has to be interrupted. This interference tends to reduce the throughput; i.e., the amount of work which is outputted from the system. With a random sampling scheme, the system is interrupted at intervals and a test run is performed. A known signal is introduced into the system or amplifier; for example, one volt and the output is monitored. If the output is within a prescribed range, the system is allowed to run. If the output is out of the prescribed range, the system is malfunctioning and corrective steps are taken.
Another problem is that if the system has intermittent failures, the random sampling technique may not detect these failures. In several applications it is important that failures or malfunctions be detected instantaneously. For example, in a reel-to-reel servo control system, any failure occurring in the amplifier means which is driving the takeup spool must be detected at once or else the media can be snapped.
Still another problem is the fact that the random sampling technique cannot continuously monitor the system for malfunction detection. In several applications, it is imperative that the malfunction device be capable of continuously monitoring or else intermittent undetected failure will be disasterous.
Finally, the prior art malfunction devices and schemes are operable over relatively narrow input signal range. However, there are several applications wherein monitoring has to be performed over relatively wide signal range (e.g., -20 volts, +20 volts) due to the signal range limitation, the prior art devices are not suitable.